Yeenoghu, together with Juiblex, is one of the two demon lords that can be summoned with chaotic own-race sacrifice. He will be peaceful then.

Though you might be prepared for Yeenoghu, you may not be prepared for what he can summon - Demogorgon - who is altogether a much bigger threat, capable of hitting through -20 AC and beyond. Since Yeenoghu has an annoying habit of hitting you and teleporting away, you may want to make him a priority kill, even if he can't hurt you, Demogorgon will.

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Yeenoghu will teleport next to you to attack you and fire magic missiles and then teleport away. He escapes upstairs to regenerate when his hitpoints are low, so an effective strategy would be to camp on the up stairs when he teleports himself off of them. He does respect Elbereth, but be sure you can deal with confusion or have it written already, since you are unlikely to write it correctly while confused.

Do not try to charm Yeenoghu; his high level and magic resistance mean you would waste more turns than in a melee fight. Also, he is covetous, so he would only become peaceful.

Yeenoghu is the subject of one of the longest-running bugs in NetHack history. In NetHack 3.1.0, the Riders were introduced. The code that handles their special attacks comes right after that which handles Yeenoghu's confusion attack[1]. Early versions had this code inserted carelessly; consequently, after Yeenoghu used his confusion attack, execution would proceed into the section that handles Death's touch[2]:

Yeenoghu reaches out with its deadly touch.--More--
Do you want your possessions identified? [yn]

A comment at the end of Yeenoghu's section read:

/* fall through to next case */

and contemporary source-divers thought from this that the behavior was deliberate. Thus for many years the bug was never reported.

The Rider code was carelessly placed between the AD_CONF section, which handles Yeenoghu's confusion attack, and the default section, where it was supposed to finish. Thus execution improperly proceeded into Death's touch attack.